Continuous PTH (cPTH) treatment causes bone loss while intermittent PTH (iPTH) treatment increases bone mass, but the reasons for the differential effects in bone of cPTH and iPTH treatment remain enigmatic. cPTH (but not iPTH) stimulates T cell production of the osteoclastogenic cytokine TNF through direct continuous PTH signaling in T cells. By contrast, T cells respond to iPTH (but not cPTH) by producing the osteogenic Wnt ligand Wnt10b. We thus hypothesize that the differential effects of cPTH and iPTH are explained by the capacity of continuous and intermittent PTH signaling in T cells to induce TNF and Wnt10b production respectively. Another effect of PTH is to downregulate the production of sclerostin a factor that decreases Wnt signaling in bone cells. Consequently, In Specific Aim 1 we will generate T cells with constitutively active PPR signaling, investigate the effects in bone of continuous PPR signaling in T cells, and determine whether PPR signaling in T cells alters the bone response to cPTH and iPTH through TNF. In Specific Aim 2 we will investigate the effects in bone of overexpression of Wnt10b in T cells, and determine whether T cell produced Wnt10b alters the bone response to cPTH and iPTH. This will be achieved by investigating whether overexpression of Wnt10b in T cells a) causes bone gain in untreated mice, and b) converts the response to cPTH from catabolic to anabolic. In addition we will c) quantify the fraction of the anabolic effect iPTH which is blocked by the overexpression of Wnt10b. This will allow us to determine the contribution of Wnt10b dependent and independent mechanisms to the anabolic activity of iPTH. In Specific Aim 3 we will determine the contribution of T cell produced Wnt10b to the sclerostin-independent bone anabolic effects of iPTH. This will be achieved by treating WT and T cell deficient mice with sclerostin antibody alone, or in combination with iPTH. In addition we will evaluate the effects of iPTH in T cell deficient SOST null and SOST transgenic mice. Lack of information about the mechanism of action of PTH in bone is a critical barrier to progress in the field of bone biology. Our proposal has the potential to decrease this barrier. If the aims of the projects are achieved scientific knowledge about the mechanism of action of PTH will be increased. Novel information about the role of T cells in the mechanism of action of PTH will provide a tangible example of the relevance of the cross-talk between lymphocytes and bone cells.